The present invention relates to a fire alarm system which processes an analog detection signal regarding smoke, temperature or the like and thereby warns of a fire on the basis of the processed data.
In conventional fire alarm systems, in general, a change in a single physical phenomenon such as smoke, heat or the like which is caused by the occurrence of fire is detected by a fire sensor, and when the detection value exceeds a preset threshold level, a fire signal is sent to the receiver and thereby warns of a fire.
However, in the case where the presence of a fire is determined by simply checking whether or not the detection value exceeds the threshold level, the occurrence of a fire is falsely determined when the detection value is over the threshold level due to causes other than a fire, for example, due to temporary noise or the like, so that a problem is caused because a spurious alarm is outputted.
On one hand, in the case of detecting smoke due to a fire, the quantity of smoke which is generated at the initial state of the actual fire is always changing with an elapsing of time due to the enlargement of the fire, an oscillating frequency which is peculiar to the flame or the like. The detection value of the smoke which is detected by the smoke detecting section of the smoke sensor also varies depending on the shape of the room or the like as well as the above-mentioned various factors. Therefore, the smoke detection value includes a number of other undesirable harmonic components in addition to the necessary inherent fundamental frequency of the smoke and is outputted from the smoke detecting section of the smoke sensor. Consequently, if the fire is determined using the detection value from the smoke sensor as is, there is a risk that a comparison is made between the threshold level and an improper detection value which deviates quite far from the inherent fundamental component of the smoke.
Since the detection value is incorrect as described above, there is a problem in that the prediction determination accuracy is deteriorated if such a conventional smoke detecting method is applied to an apparatus which is constituted in such a manner that: an analog detection signal regarding, for instance, smoke, temperature or the like which can be always obtained is sampled and converted to a number of digital data; the time interval from a point in time until the value of the detection signal becomes the threshold level is calculated using a plurality of digital data as they are by way of a differential value calculating method or a function approximation method; and the fire is predicted by checking whether or not this time interval lies within a predetermined time period.